Circuit for supplying power to a network termination unit of a message transmission system

ABSTRACT

The invention relates to a circuit for supplying power to a network termination unit ( 55 ) of a message transmission system, which is connected to a central station via a subscriber line ( 30 ). A local power supply voltage source ( 41 ), which is arranged in the network termination unit ( 55 ) and which supplies power during a normal operating state, is provided as well as a remote power supply source, which is arranged in the central station and which supplies power during an emergency operating state in the case of a failure or malfunctioning of the local power supply voltage source ( 41 ). The network termination unit ( 55 ) comprises a direct current converter with a transformer and a clocked switch ( 20 ). The primary winding ( 1 ) of the transformer is connected via the clocked switch ( 20 ) to the local power supply voltage source ( 41 ), and a connection of the primary winding ( 1 ) is connected via a first controllable switch ( 5 ) to a wire of the subscriber line ( 30 ).

[0001] The invention relates to a circuit for supplying power to anetwork termination unit of a message transmission system, which isconnected to a central station via a subscriber line. A local powersupply voltage source, which is arranged in the network termination unitand which supplies power during a normal operating state, is provided aswell as a remote power supply source, which is arranged in the centralstation and which supplies power during an emergency operating state inthe case of a failure or malfunctioning of the local power supplyvoltage source. The network termination unit includes a direct currentconverter with a transformer and a clocked switch. The primary windingof the transformer is connected via the clocked switch to the localpower supply voltage source.

[0002] In a service-integrating digital message transmission system(ISDN), the subscriber neighborhood is formed essentially by a networktermination unit, an S-interface and subscriber terminals. Power tothese terminals is supplied in a normal operating state from the networktermination via a local supply voltage and the S-interface. Theavailable power enables operation of the terminals connected to thenetwork termination.

[0003] If the local supply voltage, which for safety reasons istypically 24 VDC or 20 VAC, fails or decreases below an acceptablelevel, then the network termination automatically switches into anemergency operating state, wherein the essential functions of thenetwork termination are maintained by remotely supplied power via aremote supply voltage source arranged at a central location, for examplein an exchange. The remote supply voltage is thereby supplied via thesubscriber line that exists between the central location and the networktermination, wherein only a relatively small amount of power isavailable in the emergency operating state. The remote supply voltage istypically ±60 VDC.

[0004] Such power supplies are mainly used with ISDN systems, but canalso be employed with other transmission systems, such as for exampleADSL, HDSL, pair-gain systems (PGS) and the like.

[0005] The voltage supplied by the local supply power source as well asfrom the remote supply voltage source is converted by direct currentconverters to the values required for the various operating states.

[0006] To eliminate supply interruptions during switching from thenormal to the emergency operating state, conventional current supplycircuits have at least two separate direct current converters or aconverter with two separate primary circuits which are arranged in sucha way that when the local supply fails, the remote supply is availablewithout interruption. For this purpose, two direct current converters orat least two separate primary circuits have to continuously and inparallel supply the output voltage. Each direct current converter orprimary circuit, however, requires a separate control and acorresponding separate transformer winding which take up a relativelylarge space compared to the other components, making miniaturization ofthe circuit difficult. In addition, using two direct current convertersincreases the manufacturing costs.

[0007] It is therefore an object of the invention to provide a compactcircuit of the aforedescribed type which uses relatively little spaceand has low manufacturing costs.

[0008] This is achieved according to the invention in that a terminal ofthe primary winding is connected via a first controllable switch with awire of the subscriber line.

[0009] If the local power supply voltage source fails or hasinsufficient voltage, then the remote supply voltage can be connectedvia the controllable switch directly with the primary winding of thedirect current converter-transformer, so that the remote supply voltagecan be converted using one and the same primary circuit, making a seconddirect current converter unnecessary.

[0010] According to another embodiment of the invention, the terminal ofthe primary winding connected with the first controllable switch can beconnected via a second controllable switch with a buffer capacitor.

[0011] In this way, the network termination unit can be switched fromthe normal operating state, in which all terminals are supplied by thenetwork termination, unit into the emergency operating state, in whichonly the most important operating functions of the network terminationunit are maintained, using only a single direct current converter and/oronly a single primary circuit and only a single transformer, withoutencountering a supply bottleneck. The charge stored in the buffercapacitor is thereby capable of supplying to the primary winding of thetransformer of the direct current converter the energy necessary toswitch into the emergency operating state.

[0012] According to another embodiment of the invention, the first andsecond controllable switch can be formed by field effect transistors,which require only a small control energy.

[0013] According to another embodiment of the invention, the buffercapacitor can be connected via a charging branch with a charging voltagesource. The buffer capacitor is then continuously charged to compensatefor losses in the capacitor.

[0014] According to another embodiment of the invention, the chargingbranch can be formed by a charging resistor which is connected with thesubscriber line preferably via a blocking diode.

[0015] The charging resistor limits the charging current to the maximumallowable value of the current of the remote supply voltage.

[0016] According to another embodiment of the invention, a voltagemonitoring unit can be provided whose inputs are connected with thelocal power supply voltage source, and an output of the voltagemonitoring unit can be connected with a unit for controlling the firstand second controllable switch.

[0017] The voltage monitoring unit monitors the voltage supplied by thelocal power supply voltage source. If this voltage source fails or thevoltage drops below a minimum voltage, a control signal is supplied thatcontrols the second controllable switch, whereby the buffer capacitor isdischarged into the primary winding of the transformer. In this way,energy required for switching from the normal operation to the emergencyoperation is provided during the switch-over time.

[0018] According to another feature of the invention, an additionalvoltage monitoring unit can be provided, whose inputs are connected withthe subscriber line, whereby an output of the additional voltagemonitoring unit is connected with the unit that controls the first andthe second controllable switch.

[0019] The additional voltage monitoring unit can continuously monitorthe remote supply voltage which is required for transforming the networktermination into the emergency operating state.

[0020] In a method for switching a network termination unit from thenormal operating state into a remotely supplied state by using a circuitaccording to the invention, it is provided that the amplitude of thelocal supply voltage is continuously measured and compared with apredetermined minimum value, and that when the predetermined minimumvalue is underrun, the first controllable switch is closed and thenetwork termination unit is switched from a normal operating state intoa remotely supplied state, which preferably represents an emergencyoperating state.

[0021] By closing the first controllable switch, the potential of theremote supply voltage is supplied to the primary winding, which nowcontinues the voltage conversion. Accordingly, the network terminationunit is then remotely supplied via the same direct current converterwhich converts in the normal operating states the local power supplyvoltage source. If the network termination and the devices connectedthereto consume only a small amount of power, then the normal operationcan be maintained when switching into the remotely supplied state.Otherwise, the system has to be switched into the emergency operatingstate.

[0022] According to another method for transforming a networktermination unit from the normal operating state into the remotelysupplied state by using a circuit according to the invention, it isprovided that the amplitude of the local supply voltage is continuouslymeasured and compared with a predetermined minimum value, and that whenthe predetermined minimum value is underrun, the second controllableswitch is closed and the network termination unit is switched from anormal operating state into the remotely supplied state, whichpreferably represents an emergency operating state, wherein during theswitch-over time the charge stored in the buffer capacitor is at leastpartially supplied to the primary winding of the transformer, and thatthereafter the first controllable switch is closed.

[0023] Discharging the buffer capacitor into the primary winding of thetransformer by closing the second controllable switch makes it possibleto maintain the supply voltage for the network termination unit duringthe switch-over into the emergency operating state.

[0024] According to another embodiment of the invention, the remotesupply voltage can be decreased in the normal operating state to apredetermined value and increased to its full value when the local powersupply voltage source fails or has an insufficient voltage or forrecharging the buffer capacitor.

[0025] In this way, the remote supply voltage can be kept low in thenormal operating state and increased only to the required value whenneeded. This, on the one hand, reduces the energy consumption in thesubscriber line due to an excessive remote supply voltage and, on theother hand, reduces the danger associated by a high remote supplyvoltage.

[0026] According to yet another embodiment of the invention, thesubscriber line can be forcibly switched into the emergency operatingstate for maintenance and test purposes and thereafter returned to thenormal operating state. In this way, the functionality of the circuit ofthe invention can be checked.

[0027] Exemplified embodiments of the invention will be describedhereinafter in detail with reference to the drawings. It is shown in

[0028]FIG. 1 a schematic circuit diagram of the embodiment of thecircuit of the invention, and

[0029]FIG. 2 a schematic circuit diagram of another embodiment of thecircuit of the invention.

[0030]FIG. 1 shows a circuit for supplying power to a networktermination unit 55 of a message transmission system which is connectedvia a subscriber line 30 with a central location of the messagetransmission system (not shown), for example an exchange. Such circuitsare used, for example, in ISDN transmission systems, but can also beused for supplying power in comparable systems, such as for exampleADSL, HDSL, PGS and the like.

[0031] The network termination unit 55 represents the interface betweenthe data transmission of the subscriber line 30 and subscriber terminals(not shown). The network termination unit 55 is powered via a localpower supply voltage source 41, for example a DC or AC power supply,which operates a direct current converter comprised of a transformer, aclocked switch 20 and a clock circuit controlling the clocked switch,for example a pulse width modulator 2. The primary winding 1 of thetransformer is connected via the clocked switch 20 with local powersupply voltage source 41 whose voltage is chopped by the switch 20, sothat on the primary side a series of square pulses is produced.Accordingly, a periodic secondary voltage according to the selectedturns ratio of the transformer is generated in the secondary winding 1′which is inductively coupled with the primary winding 1. The secondaryvoltage is rectified and filtered by a filter and rectifier circuit 56for supplying to the network termination unit 55.

[0032] As long as of the local supply voltage source 41 maintains itsvoltage, all subscriber terminals can be supplied via the networktermination unit 55, which is then in the normal operating state. Thedata traffic of the network termination unit 55 to the exchange occursby symmetric feed of the data into the subscriber line by a transformer13 indicated in FIG. 1. The connection to the subscriber terminals isnot shown in FIG. 1 for sake of clarity.

[0033] The voltage generated by the supply voltage source can decreaseor completely fail for various reasons. If this happens, the networktermination unit 55 and all control units and additional units which arerequired for the emergency operation, switch from the normal operatingstate into the emergency operating state wherein only the most importantoperating functions are maintained. Power is then supplied by the remotesupply voltage source located at the exchange.

[0034] To ensure a smooth transition from the local supply to the remotesupply, it is provided according to the invention that a terminal of theprimary winding 1 is connected via a second controllable switch 4 with abuffer capacitor 3 and via a first controllable switch 5 with a wire ofthe subscriber line.

[0035] In the normal operating state, the amplitude of the local supplyvoltage is continuously measured and compared with a predeterminedminimal value. According to the embodiment of the invention depicted inFIG. 1, the supply voltage in the normal operating state is, forexample, 24 VDC or 24 VAC, with an exemplary power consumption ofapproximately 9 W. The remote supply voltage is, for example, set to ±60V. Other values for the voltage or power can be selected.

[0036] If the voltage decreases below a predetermined minimal value,e.g., 21.5 VDC, then the second controllable switch 4 is closed and thenetwork termination unit 55 as well as the control (not shown) areswitched from a normal operating state into an emergency operatingstate. Thereafter, the first controllable switch 5 is closed, wherebythe remote supply voltage is applied to the primary winding 1, which isthen chopped by the clocked switch 20 and transmitted into the secondarywinding 1′, thereby maintaining the emergency operating state via theremote supply voltage.

[0037] In the normal operating state, the primary winding 1 of thedirect current converter-transformer converts the voltage of the localpower supply voltage source 41, with the primary winding 1 convertingthe remote supply voltage after the switch-over. In this way, only asingle transformer and a single primary circuit are required for thedirect current conversion, which saves considerable space for thecircuit of the invention.

[0038] When the second controllable switch 4 is closed, the chargestored in the buffer capacitor 3 is applied to the primary winding 1 soas to maintain the direct current conversion for a short period of timeto enable switch-over into the emergency operating state. Thereafter,the total remote supply voltage is applied via the first switch 5 to theprimary winding, without interruption in the supplied power.

[0039] The two controllable switches 4, 5 as well as the controllableswitch 20 are implemented in FIG. 1 as FET transistors, but can also beimplemented as other forms of conventional controllable switchingelements.

[0040] For detecting a failure of the supply voltage source 41, avoltage monitoring unit 15, which is implemented for example as acomparator circuit, is provided, whose inputs are connected with thelocal power supply voltage source 41. The output of the voltagemonitoring unit 15 is connected with a unit for controlling the firstand second controllable switch 5, 4, with the unit being shown in FIG. 1as two separate control units 7, 8.

[0041] The buffer capacitor 3 is connected with the subscriber line 30via a charging resistor 17 and a blocking diode 68. The capacitor 3 canalso be charged without the charging resistor 17, which is necessaryonly if the storage capacity of a buffer capacitor 3 is very large orthe charging voltage, in the illustrated case the remote supply voltage,is only capable of charging capacitors up to a certain maximumcapacitance. The buffer capacitor 3 can also be charged using anothervoltage source. The value of the charging resistor 17 depends on theremote supply voltage and the maximum remote supply current availablefor charging the capacitor 3.

[0042] In order to monitor the remote supply voltage, an additionalvoltage monitoring circuit 14 is provided, whose inputs are connectedwith the subscriber line 30, wherein an output of the additional voltagemonitoring unit 14 is connected with a unit for controlling the firstand second controllable switches 5, 4.

[0043] As long as the local power supply voltage source 41 supplies asufficiently large supply voltage, the remote supply voltage is appliedto the series connection of charging resistor 17 and buffer capacitor 3via the subscriber line 30, with the buffer capacitor 3 maintained in acharged state via the charging resistor 17.

[0044] When the local supply voltage falls below a predetermined minimalvalue, then the voltage monitoring unit 15 controls an optocoupler 9,causing the potential on a resistor 52 to change, which is communicatedto a control system (not shown) as an error signal. The control systemcontrols an optocoupler 10, which closes via a switch control unit 8 thesecond controllable switch 4, whereby the buffer capacitor 3 isdischarged via the primary winding 1, maintaining the current conversionfor a short time, if the voltage monitoring unit 14 detects a specifiedremote supply voltage on the subscriber line 30. The remote supplyvoltage then generates via an optocoupler 12 a control signal on aresistor 51 which is supplied to the control system (not shown). Afterthe discharge time of the buffer capacitor 3, the control systemcontrols an optocoupler 11, which closes via an additional switchcontrol unit 7 the first controllable switch 5 and thereby applies theremote supply voltage to the primary winding 1. The additional switchcontrol unit 7 can also be controlled directly via the optocoupler 12 soas to close the first controllable switch 5.

[0045] According to another embodiment of the circuit, the optocoupler 9can also directly control the switch control unit 8 which affects theswitch position of the second controllable switch 4.

[0046] As long as neither the local supply voltage nor the remote supplyvoltage reach the required value, the control system is reset to aninitial state.

[0047] The other switch components depicted in FIG. 1 are of lesserimportance. For example, the bridge rectifier 32 makes the systemindependent of the polarity, whereas capacitors 21, 33, 42, 43 and 45operate as termination and filter capacitors.

[0048] Diode 31 operates as a blocking diode with respect to the remotesupply voltage. The supply input of the pulse width modulator 2 is alsoprotected by blocking diodes 18, 19. The pulse width modulator 2 issupplied during the startup phase via the remote supply voltage anddiode 18, whereby a limiting resistor 52 limits the remote supplycurrent to a maximum allowable value. During the normal operation, thepulse width modulator 2 is powered via the diode 19 or via a speciallyprovided winding of the transformer (not shown).

[0049] When the switch of the invention is first turned on, the processflow is as follows:

[0050] 1) Start-Up with Available Local Supply

[0051] Initially, the voltage monitoring unit 15 indicates that a localsupply voltage is present, whereafter the pulse width modulator 2 isstarted. After the voltage is built up, the control system is activated.The buffer capacitor 3 is charged via the remote supply voltage that ispresent on the subscriber line 30. When the local supply voltage failsor has an insufficient voltage, the second controllable switch 4 isclosed and the operation is switched to the emergency operating state.When the switch-over is complete after, for example, 10 to 20milliseconds, the first controllable switch 5 is closed and the remotesupply voltage is applied to the primary winding 1. As soon as a localsupply voltage is again available with full amplitude, the firstcontrollable switch 5 is opened first, whereafter the secondcontrollable switch 4 is opened.

[0052] 2) Start-Up with Unavailable Local Supply

[0053] Initially, the voltage monitoring unit 15 indicates that theremote supply voltage is present. Because the local supply voltage isnot available, the control system switches into the emergency operatingmode. The buffer capacitor 3 is discharged via the charging resistor 17.If a defined remote supply voltage is present, then the firstcontrollable switch 5 is closed and the supply voltage is applied to theprimary winding 1.

[0054] If the voltage of the remote supply voltage can be altered, thenit becomes possible to lower the remote supply voltage in the normaloperating state to a predetermined value and to increase the remotesupply voltage to its full value only when the local power supplyvoltage source fails or has an insufficient voltage or for rechargingthe buffer capacitor 3.

[0055] Because the remote supply voltage increases relatively slowly,the buffer capacitor 3 has to be dimensioned so that the required timeis included in the discharge time of the buffer capacitor 3.

[0056] In addition, the invention also includes an embodiment without abuffer capacitor 3. In this embodiment, the second controllable switch 4is eliminated and only one terminal of the primary winding is connectedto a wire of the subscriber line 30 via the first controllable switch 5.In the normal operating state, the amplitude of the local supply voltageis continuously monitored and compared with a predetermined minimumvalue. When the predetermined minimum value is underrun, the firstcontrollable switch 5 is closed and the network termination unit isswitched from the normal operating state into the emergency operatingstate.

[0057] Switchback from the Emergency Operating State to Normal OperatingState

[0058] When the full supply voltage returns, the voltage monitoring unit15 reports this to the control system, causing the first controllableswitch 5 and, if necessary, also the second controlled both switch 4 tobe switched off. Both switches 4, 5 have to be open for the currentsupply to switch from the emergency operating mode to the normaloperating mode.

[0059] The subscriber line 30 is forcibly switched into the emergencyoperating state for maintenance and test operations. For this purpose,an additional controllable switch (not shown in FIG. 1) can be providedfor interrupting the local supply voltage.

[0060] In the embodiment of FIG. 2, the power consumption of the networktermination unit and of all pertaining units is so small that full powercan be supplied from both the local supply voltage 41 and the remotesupply voltage. One terminal of the primary winding 1 is herebyconnected via the first controllable switch 5 with a wire of thesubscriber line 30. The system can operate in the following two ways:

[0061] 1) Powered via the Local Supply

[0062] The voltage monitoring units 14 and 15 monitor the remote supplyvoltage and the local supply voltage. If the local supply voltage source41 fails or has an insufficient voltage, then the first switch 5 isclosed when a sufficient remote supply voltage is detected, and theremote supply voltage is smoothly switched to the primary winding 1. Abuffer capacitor 3 is therefore not required. As soon as the full localsupply voltage is available, the voltage monitoring unit 15 reports thisto the control system, causing the first controllable switch 5 to beswitched off.

[0063] 2) Powered via the Remote Supply Voltage

[0064] If a local supply voltage is not present, then the firstcontrollable switch 5 is closed, and the remote supply voltage isswitched to the primary winding 1. The voltage monitoring unit 14continuously checks if a sufficiently large remote supply voltage ispresent on the subscriber line 30. If the local power supply voltagesource 41 is to be connected at full voltage, then the voltagemonitoring unit 15 reports this to the control system, causing the firstcontrollable switch 5 to be switched off. The operation then continuousvia the local power supply.

1. Circuit for supplying power to a network termination unit of amessage transmission system, which is connected to a central station viaa subscriber line, wherein there is provided a local power supplyvoltage source, which is arranged in the network termination unit andsupplies power during a normal operating state, and a remote powersupply source, which is arranged in the central station and suppliespower during an emergency operating state in the case of a failure ormalfunctioning of the local power supply voltage source, and wherein thenetwork termination unit includes a direct current converter with atransformer and a clocked switch and wherein the primary winding of thetransformer is connected via the clocked switch to the local powersupply voltage source, characterized in that a terminal of the primarywinding (1) is connected via a first controllable switch (5) to a wireof the subscriber line (30).
 2. Circuit according to claim 1,characterized in that the terminal of the primary winding (1) connectedwith the first controllable switch (5) is connected via a secondcontrollable switch (4) with a buffer capacitor (3).
 3. Circuitaccording to claim 1 or 2, characterized in that the first and secondcontrollable switches are formed by field effect transistors (5, 4). 4.Circuit according to claim 2 or 3, characterized in that the buffercapacitor (3) is connected via a charging branch (17) with a chargingvoltage source.
 5. Circuit according to claim 4, characterized in thatthe charging branch is formed by a charging resistor (17) which isconnected with the subscriber line (30) preferably via a blocking diode(16).
 6. Circuit according to one of the claims 1 to 5, characterizedthat a voltage monitoring unit (15) is provided, whose inputs areconnected with the local power supply voltage source (41), and that anoutput of the voltage monitoring unit (15) is connected with a unit forcontrolling the first and second controllable switches (5, 4). 7.Circuit according to one of the claims 1 to 6, characterized in that anadditional voltage monitoring unit (14) is provided, whose inputs areconnected with the subscriber line (30), and that an output of theadditional voltage monitoring unit (14) is connected with the unit forcontrolling the first and the second controllable switches (5, 4). 8.Method for transferring to a network termination unit from a normaloperating state to a remotely supplied state by using a switchingcircuit according to claim 1, characterized in that the amplitude of thelocal supply voltage is continuously measured and compared with apredetermined minimum value, and that when the predetermined minimumvalue is underrun, the first controllable switch (5) is closed and thenetwork termination unit (55) is switched from a normal operating stateto a remotely supplied state, which preferably represents an emergencyoperating state.
 9. Method for transferring a network termination unitfrom a normal operating state to a remotely supplied state by using aswitching circuit according to claim 2, characterized in that theamplitude of the local supply voltage is continuously measured andcompared with a predetermined minimum value, and that when thepredetermined minimum value is underrun, the second controllable switch(4) is closed and the network termination unit (55) is switched from anormal operating state to the remotely supplied state, which preferablyrepresents an emergency operating state, wherein during the switchingtime the charge stored in the buffer capacitor (3) is at least partiallysupplied to the primary winding (1) of the transformer, and thatthereafter the first controllable switch (5) is closed.
 10. Methodaccording to claim 8 or 9, characterized in that the remote supplyvoltage is decreased in the normal operating state to a predeterminedvalue and increased to its full value, when the local power supplyvoltage source (41) fails or has an insufficient voltage or forrecharging the buffer capacitor (3).
 11. Method according to claim 8, 9or 10, characterized in that the subscriber line (30) is forciblyswitched for maintenance and test operations into the emergencyoperating state and thereafter returned again to the normal operatingstate.